This bc1 complex and its chloroplast sister the b6f complex are essential for the establishment of the transmembrane proton gradient that is necessary for the conversion of carbohydrate and photon derived energies into high-energy chemical bonds. The bc1 complex is well studied from many organisms, with several crystal structures solved and a plethora of amino acid point mutations characterized. From this body of work it has been established that a large domain movement is critical for the catalysis of the enzyme. It is unclear however what initiates this movement and how or if it is controlled by the redox events that take place n the various regions of the complex. What is clear is that the movement of the region, which contains the paramagnetically visible [2Fe2S] cluster, is integral to the processes related to the modified Q-cycle. This cycle is the cog that allows for the efficient proton-pumping characteristic of this enzyme complex. We are proposing to utilize the orientationally dependent paramagnetic properties of the [2Fe2S] cluster in this complex in oriented samples of Rhodobacter capsulatus membranes to monitor the effects of mutations in various regions of the complex to identify the roles that various regions play in the control of the movement of the FeS protein region. We will also utilize the spectroscopic technique of electron paramagnetic resonance on oriented samples to probe the interaction of the FeS head domain and the cytochrome b containing subunit with the specific aim of identifying possible mechanisms of domain movement control during the catalytic cycle. The results obtained in this work should begin to explain the anomalous data obtained with mutations made in seemingly unrelated regions to the catalytic impairments observed as well as open the door to more advanced interpretations of mutational data in the absence of crystal structure data for the multitudes of amino acid changes made.